linux-kernel - Re: [PATCH -mm][resend] Disable CPU hotplug during suspend

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Date:	Sat, 29 Jul 2006 16:40:31 -0500
From:	Nathan Lynch <ntl@...ox.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Andrew Morton <akpm@...l.org>, LKML <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>
Subject: Re: [PATCH -mm][resend] Disable CPU hotplug during suspend

Rafael J. Wysocki wrote:
> On Saturday 29 July 2006 00:40, Nathan Lynch wrote:
> 
> > But maybe I'm misunderstanding the motivation for using cpu 0 here.  I
> > had assumed it was because on i386 (and others?) the BSP can't be
> > offlined.  Is there some other reason?
> 
> Yes.
> 
> First, the arch-dependent suspend code assumes implicitly that it will be
> running on the BSP, so some strange things may happen if it doesn't.
> 
> Second, we have to make sure that this function will always leaves the
> same CPU online.  It's a bit difficult to explain, but I'll do my best.
> Suppose that disable_nonboot_cpus() exits running on CPU1, assuming it's
> possible.  Then the system memory state saved in the suspend image will
> reflect this situation.  Now the resume code will almost certainly run on the
> BSP (say it's CPU0), but when the system memory is restored from the suspend
> image the kernel will think it's running on CPU1.
> 
> In the last patch I send yesterday I made disable_nonboot_cpus() check if the
> first present CPU, first_cpu(cpu_present_map), is online, try to bring it up
> if not and migrate itself to it before the loop over all online CPUs is run.
> 
> I think that's general enough.

I see, thanks for the explanation.

It doesn't look like SMP swsusp would work reliably on platforms where
there's a possibility of the cpu maps in the resume and saved images
not matching (e.g. ppc64 logical partitions, where cpu 0 could be
removed before suspending).  But I guess that's largely a theoretical
concern at this time. ;)

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